This invention is related to the separation of a mixture into a first fraction relatively rich in a first component and a second fraction relatively rich in a second component. More specifically, this invention is related to the separation of petroleum processing feed streams, especially filtrate, extract or raffinate streams from dewaxing or extraction zones.
In the separation of a mixture, one or more unit operations may be utilized. Frequently, the particular unit operation is selected after a consideration of several variables, including the desired purity of the separated fractions, capital cost of the equipment, equipment reliability and operating costs. In the production of lube oils from base feedstock, the feedstock is processed to remove undesired aromatics and waxy compounds. Typically, these compounds are removed by extractive operations in which a solvent having the desired affinity for one of the feedstock components is passed concurrently, cross-currently or countercurrently to the feedstock to effectuate the separation. For example, in the separation of aromatics from lube oil feedstock, the feedstock may be contacted with a solvent such as phenol, furfural, acetone, and N-methyl pyrrolidone each of which is selectively miscible with the aromatics to be removed. In the separation of waxy compounds, the lube oil feedstock frequently is contacted with a solvent or solvent mixture such as methyl ethyl ketone (MEK) methyl isobutyl ketone (MIBK), methyl ethyl ketone/toluene, or acetone/methylene chloride which selectively removes the waxy compounds. Previously, efforts have been directed at the subsequent processing of the extract and raffinate streams from the extraction zone to separate and recover the solvent, as well as to further purify the lube oil base, which typically comprises the raffinate.
U.S. Pat. No. 2,232,722 is directed at a dewaxing process in which the solution is chilled and decanted to separate the filtrate into two liquid phases for recycle and/or additional processing, such as by distillation, an energy intensive process. Additional solvent may be added to the solution to facilitate the phase separation.
U.S. Pat. Nos. 3,725,257 and 3,985,644 both are directed at the extractive separation of an aromatic-rich stream from a petroleum fraction. The extract then is cooled and separated into an aromatic-rich hydrocarbon phase and a solvent-rich phase. The solvent-rich phase is recycled either to an intermediate point or to the top of the extractor while the hydrocarbon-rich phase is passed to a distillation zone. This process is deficient in that the solvent returned to the extractor contains significant quantities of aromatic hydrocarbons. Thus, recycling this recovered solvent to the top of the extractor will require significantly more solvent for effective removal of aromatics from the aromatic-containing feedstock.
U.S. Pat. No. 2,754,249 also discloses the extraction of a hydrocarbon fraction to remove non-paraffinic compounds. The extract is de-oiled by the addition of a material capable of reducing the solubility of the extracted hydrocarbons in the solvent. The extract subsequently is decanted, with the layer containing solvent and anti-solvent distilled to separate the anti-solvent. The solvent, substantially devoid of anti-solvent, then is mixed with solvent recovered from the raffinate phase and recycled to the extraction system. This process depends upon the use of large amounts of anti-solvent to effect the required separation. Thus, the solvent-rich phase must be distilled to remove large quantities of anti-solvent, resulting in this process being energy intensive.
U.S. Pat. No. 3,556,991 is directed at a method for removing aromatic hydrocarbons from a petroleum fraction by solvent extraction. In this process, the extract is passed through a membrane separation zone having a membrane permeable to the aromatic hydrocarbon but not to the solvent. The retentate solvent then is directed through a distillation zone to further purify the solvent. Although the membrane separation zone removes a fraction of the solvent, significant quantities of the solvent must be removed during the distillation step.
Accordingly, it is desirable to provide a process which is reliable, and offers low operating and maintenance costs.
It is also desirable to provide a process which requires relatively low capital costs and which may be retrofitted to existing separation facilities for improving production rates and/or product quality.
The subject invention is directed at the discovery that decantation and separation of a mixture prior to passage through a membrane separation zone improves the flux and/or degree of separation of the permeate. This procedure thus, may decrease the number of membrane separation zones required for a given throughput and/or reduce the load placed on downstream separation facilities, such as a distillation zone.